Challenges in prognostication of FIV-positive patients

Posted on July 1st, 2013

Veterinarian Julia Beatty with FIV positive rescue cat Bob.

A Sydney University study published in the Journal of Veterinary Internal Medicine found that the presence of haematologic and biochemical abnormalities could not be relied upon to determine clinical progression of FIV in sick feline patients, and that FIV-positive status alone did not have a negative impact on survival.

The authors set out to compare signalment, complete blood count and biochemistry panel, major clinical problem and survival between client-owned FIV-positive and uninfected domestic cats. The retrospective study, one of the largest of its kind, involved 520 cats tested for FIV.

Whilst reasonably straightforward to diagnose, often with an in-house antibody detection kit, feline experts continue to puzzle about how FIV contributes to disease status. Naturally infected cats present with a range of clinical signs including stomatitis, cachexia, atypical, refractory or recurrent infections, neurologic signs, persistent cytopaenias and immune-mediated disease – but these problems are often seen in FIV-negative cats too. With the exception of a small subset of lymphomas, AIDS-defining illnesses are not recognised for FIV. Additionally, many FIV-positive cats remain asymptomatic with a normal life expectancy.

In experimental studies of FIV infection, cats have shown progressive aberrations in measures of immune function such as lymphocyte subset counts and mitogen responsiveness, but these changes are rarely associated with clinical signs.

The overall prevalence of FIV in the study population was 14.6 per cent, with mixed-breed, older male cats at highest risk. However, the authors found that while lymphoid malignancies were slightly more common in the FIV-infected cohort, haematologic abnormalities, biochemical abnormalities or both were common in FIV-infected and non-infected cats.

“We found that a lot of the FIV-positive cats were anaemic and leukopaenic – but so were the non-FIV positive cats,” group leader Julia Beatty said.

“The big challenge with FIV-positive, sick cats is to determine whether FIV is actually contributing to the clinical signs of the animal.”

The study’s findings suggest that veterinarians can’t rely on haematology and biochemistry results to determine whether FIV is contributing to the disease status of a particular cat.

Beatty pointed out that monitoring FIV progression is not equivalent to monitoring HIV progression in human patients.

“In feline patients we don’t have surrogate markets like CD4 counts used in humans with HIV – and our study has now found that the CBC and biochem panel doesn’t tell us as much as perhaps we previously thought it did.”

Most previous studies which looked at CBC and biochemistry results in FIV-positive cats had not utilised FIV-negative controls.

“Our study highlights the importance of using a control sample when ascribing clinical importance to changes in CBC and biochemistry parameters,” Beatty said.

For veterinarians in practice, that means that clinicians should prognosticate without regard to FIV-status.

“If the cat is intractably azotaemic, cachectic or not responding to antibiotics, these may indicate a poor prognosis, but that is the case regardless of the FIV status of the cat,” Beatty said.

There is some good news for owners of FIV-positive cats. The study confirmed that survival time was comparable for FIV-infected and non-infected cats.

“This contradicts a still widely-held belief that FIV infection necessarily reduces lifespan, but is consistent with recent case control studies,” Beatty said.